Breast cancer is one of the most common malignancies in the United States, affecting one in nine women. Growth factors and their receptors play an essential role regulating the proliferation of epithelial cells. Abnormalities in their expression and action contribute to the progression and maintenance of the malignant phenotype. For example: overexpression of the c-erbB-2/HER2 gene product is frequently associated with an aggressive clinical course, reduced disease-free survival, and increased metastasis in human breast cancer. Similarly, transactivation of HER2 receptor by the binding region of heregulin (HRG) to its receptors (HER3 or HER4) promotes the development of the hormone-independent and more aggressive phenotype. The focus of this proposal is to establish the molecular mechanism(s) by which HRG participate in the development of the more invasive phenotype in breast cancer cells. The rationale behind this proposal is based on the recent observations that HRG-stimulation of human breast cancer cells leads (i) to the induction of F-actin (ii) a decreased association between E- cadherin and beta-catenin at the cell-cell interface, (iii) increased physical association between beta-catenin and the small GTPase cdc42, (iv) activation and a redistribition of p21 activated kinase (PAK) at the leading edge and (v) the development of lamellipodia/filapodia. In addition, these molecular changes are accompanied by a stimulation of trans-migration of breast cancer cells through a porous membrane and increased extracellular secretion of bioactive metalloprotease-9. These findings suggest that (i) cytoskeletal complexes that control cell to cell adhesion and cell motility may have a significant role in the cell migration and invasion functions of HRG in breast cancer cells and (ii) cross talk occurs between HRG signaling and the pathways that control the cell migration and adhesion phenotype of breast cancer cells, and thus may lead to enhance invasion. Their central hypothesis is that "activation of the HRG-signaling pathway(s) may influence the function and/or expression of specific gene products that regulate cell adhesion, stress fiber formation, membrane ruffling and cell migration and, therefore, may contribute to the development of the cell migration/ invasive phenotype of breast cancer cells." The overall goals of this proposal are (1) to establish the cell migration function of HRG (Aim1). (2) To establish a functional link between HRG action and the modulation of cytoskeletal complexes and cell to cell interactions, and to determine the signal transduction pathways that regulate the development of lamellipodia/filapodia and cell migration (Aims 2 and 3). (3) To establish the signaling pathways that may be responsible for the upregulation of MMP-9 in HRG treated breast cancer cells (Aim4). One of the unique/innovative aspects of our proposal is that we will delineate at which point in the signaling cascades HRG generated signals are bifurcated in favor of cell migration by influencing the development of filapodia/lamellipodia or toward invasion by inducing MMP-9 expression. The significance of the proposed research is that it may lead to an enhanced understanding of the fundamental molecular processes involved in HER2/HRG regulate cell migration and invasion functions. It may also provide new intracellular targets for the development of therapeutic agents to inhibit breast cancer progression.